The invention has wide-ranging application across many fields of industry. It is particularly suited to pressure measurement in harsh or dynamic environments that would preclude many other pressure sensors. These applications include, but are not limited to:                monitoring engine pressure (cars, aircraft, ships, fuel cells)        sensors for high speed wind tunnels        sensors to monitor explosions        sensors for boilers        sensors for dish-washing machines        sensors for irons (both domestic and industrial)        sensors for other steam based machines where overpressure can lead to destruction and loss of life        
However, in the interests of brevity, the invention will be described with particular reference to a tire pressure monitor and an associated method of production. It will be appreciated that the Tire Pressure Monitoring System (TPMS) described herein is purely illustrative and the invention has much broader application.
Transportation Recall Enhancement, Accountability and Documentation (TREAD) legislation in the United States seeks to require all U.S. motor vehicles to be fitted with a tire pressure monitoring system (TPMS). This is outlined in U.S. Dept. of Transportation, “Federal Motor Vehicle Safety Standards: Tire Pressure Monitoring Systems; Controls and Displays”, US Federal Register, Vol. 66, No. 144, 2001, pp. 38982-39004. The impetus for this development comes from recent Firestone/Ford Explorer incidents which led to a number of fatal accidents. A careful assessment of tire inflation data found that approximately 35% of in-use tires are under inflated, whilst an assessment of the effect of a TPMS found that between 50 to 80 fatalities, and 6000 to 10,000 non-fatal injuries, per annum could possibly be prevented. This is discussed in U.S. Dept. of Transportation, “Tire Pressure Monitoring System,” FMVSS No. 138,2001. European legislation also appears likely to require the fitting of a TPMS to increase tire life, in an effort to reduce the number of tires in use by 60% in the next 20 years, so as to minimise the environmental impacts.
Two different kinds of TPMS are currently known to be available in the marketplace. One kind of TPMS is based on differences in rotational speed of wheels when a tire is low in pressure. The asynchronicity in rotational speed can be detected using a vehicle's anti-braking system (ABS), if present. The second kind of TPMS measures tire pressure directly and transmits a signal to a central processor. FIG. 1 (prior art) illustrates a schematic of a typical pressure measurement based TPMS 10. Sensors 12, provided with a transmitter, measure pressure in tires 13 and transmit a signal 14 to antenna 16. The data can then be relayed to a receiver 15 and processed and displayed to a driver of the vehicle 17 on display 18.
Table 1 lists some presently known TPMS manufacturers/providers. Motorola and Pacific Industries have each developed a TPMS, whilst other companies listed in Table 1 act as suppliers for TPMS manufacturers, including some automobile producers that install their own TPMS.
TABLE 1Pressure sensor manufacturers involved in TPMS.CompanySupplier toType of SensorMotorolaMotorolaCapacitancePacific IndustriesPacific IndustriesPiezoresistiveSensoNorSiemens, TRW, Beru,PiezoresistivePorsche, BMW, Ferrari,Mercedes, ToyotaSiemensGoodyearPiezoresistiveTransense TechnologiesUnder developmentSurface AcousticWaveTRW/NovasensorSmartire, Michelin,PiezoresistiveSchrader, Cycloid
There are two main types of pressure sensor; resistive or capacitive. Both types of these sensors rely on deflection of a membrane under an applied pressure difference. One side of the membrane is exposed to internal pressure of a tire while the other side of the membrane forms one wall of a sealed cavity filled with gas at a reference pressure.
The resistive-type sensors typically employ silicon-based micro-machining to form a Wheatstone bridge with four piezoresistors on one face of the membrane. The sensor responds to stress induced in the membrane. For capacitive-type sensors, the membrane forms one plate of a capacitor. In this case, the sensor responds to deflection induced in the membrane. Preferably, the responses should be linear with pressure, for predictability, up to at least a critical point.
Transense Technologies, listed in Table 1, have developed a different type of sensor, based on surface acoustic wave detection. This sensor relies on interferometric measurement of the stress-induced deflection of a reflective membrane. A fibre-optic cable both transmits and receives laser light, with one end of the fibre-optic cable being inserted into the interferometer. This system is discussed in Tran, T. A. Miller III, W. V., Murphy, K. A., Vengsarkar, A. M. and Claus, R. O., “Stablized Extrinsic Fiber Optic Fabry-Perot Sensor for Surface Acoustic Wave Detection”, Proc. Fiber Optic and Laser Sensors IX, SPIE vol. 1584, pp 178-186, 1991.
Presently, there are also a variety of different kinds of deployment means for sensors in a TPMS, including valve cap and valve stem based systems, systems with the sensor mounted on the wheel rim or wheel hub, and also a tire-wheel system developed by an alliance of several tire manufacturers which has a sensor embedded in the wheel frame itself. These different kinds of deployment in TPMS are listed in Table 2.
TABLE 2Specifications of TPMS in production.WarningCompany/Type ofLevelAccuracyGroupSystemFitted to(psi)(psi)SamplingBeruWheel RimAudi, BMW,user set1every 3 sec,Mercedestransmittedevery 54 secCycloidWheel CapFord,18130 sec/10 min(pump)GoodyearFleetValve Capheavy2013.5 secvehiclesJohnsonValve StemAM  19.9115 minMichelin/PAXRenault,???Goodyear/Pirelli/SystemCaddillacDunlopMotorolaWheel RimAM??6 secOmronValve StemAM???Pacific IndustriesValve StemAM20.3/user  1.815 sec/10 minsetSchraderValve StemCorvette,222%?Peugeot,CadillacSmartireWheel RimAston?  1.56 secMartin,Lincoln, AMAM = products fitted to a vehicle after vehicle purchase (After Market).
To increase battery life, most TPMS are in stand-by mode for the majority of time, only operating at set intervals. The U.S. legislation requires the system to alert the driver within a set time of detecting significant tire under-inflation conditions. It also requires a warning light to signal when the tire is either 20% or 25% under-inflated. Most of the devices presently available in the market are accurate to within ±1 psi, which represents ±3% for a tire pressure of 30 psi. More generally, the sensor should perform in a harsh environment, with temperatures up to 130° C. and accelerations of 1000 g or more. Tire pressure increases and decreases in response to corresponding changes in temperature. Most systems presently available include a sensor to account for thermally induced changes in tire pressure sensor sensitivity (Menini, Ph., Blasquez, G., Pons, P., Douziech, C. Favaro, P. and Dondon, Ph., “Optimization of a BiCMOS Integratetd Transducer for Self-Compensated Capacitive Pressure Sensor,” Proc. 6th IEEE Int. Conf. Electronics, Circuits and Systems, Vol 2, pp. 1059-1063, 1999).
Fabricating a MEMS pressure sensor on, and through, a silicon wafer substrate, requires etching from both sides of the wafer. The reference chamber needs to be filled with sacrificial material so that the membrane can be deposited on top. Then, the final etches removes the sacrificial material in the chamber with a deep back etch (etching from the reverse side of the wafer through to the front side). The holes from the deep etch need to be closed to seal the chamber. Typically, this is done by securing another wafer to the backside of the main wafer.
Resistive and capacitative pressure sensors rely on the reference fluid pressure remaining constant. If the reference fluid leaks, the sensor suffers calibration drift. The permeability of the seal closing the backside of the wafer is the main cause of leakage. It is a simple matter to adhere a separate wafer to the backside of the main wafer in order to seal the etch holes. However, the air permeability of polymeric adhesive would result in the reference cavity pressure equilibrating with the tire pressure in about 100 days. This is clearly unsuitable.
Another option is to over-mold the sensor with plastic to seal the reference chamber. Plastic has a permeability 10 times less than adhesive but it will appreciated that this is still too high. The calibration drift would be unacceptable within a year.
The reference to any prior art in this specification is not, and should not be taken as, an acknowledgment or any form of suggestion that such prior art forms part of the common general knowledge.